The role of cytoskeletal elements in the final stages of the morphogenesis of murine mammary tumor virus (MuMTV) will be investigated using anti-microfilament and anti-microtubule agents. Since actin is present in purified MuMTV, the ability of virus nucleoids and membranes to interact with actin will be investigated by electron microscopy. The existence of heterogeneity in the actin of mammary epithelial cells will be investigated using isoelectric focusing and if heterogeneity is found, it will be possible to ascertain whether virus production has purified a particular form of actin and whether the ratio of actins found in normal and transformed cells is different. In the process of purifying actins by several methods, any actin binding proteins present will be identified and purified. The distribution of these proteins in normal and transformed cells and in MuMTV will be investigated by immunologic methods.